igbvf: resolve "frees DMA memory with different size" warnings

This change resolves some warnings seen with DMA debugging enabled in which
we were mapping skb->data with size + NET_IP_ALIGN and unmapping it with
just size.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>